Teachers tends to overemphasize verbal learning, where objects, particularly mathematical objects, are taught as complete abstract entities. Computer tools like Modellus can help change the emphasis in a direction where mathematical objects are objects-to-think-with, objects to study interactively. Learning can then become concrete - mathematical objects behave like "real" objects and simultaneously maintain their abstractness: they represent the essential features of a phenomenon, not the whole phenomenon. Objects in Modellus can be classified as a new class of objects: concrete-abstract objects. Concrete in the sense that they can be manipulated directly with a computer and abstract in the sense that they are representations of mathematical constructs.